Aspect ratio when playing older 4:3 games on a wide screen

My problem is that when I play an older game like Stronghold Crusader or other games that use a 4:3 aspect ratio, it gets pulled apart to 16:9 on my widescreen monitor.

Is there a way to prevent this? I don't mind having a black bar on either side, as long as the game looks correct.

For most Acer monitors this can be adjusted trough the monitor's menu.

Press the menu button, then press the < / > buttons to navigate to the settings tab, and adjust the "wide mode" setting.

Reference: Acer S235HL user manual, page 24

This is often a setting in the graphics driver menu, but the exact location will vary.

For NVIDIA cards, open the NVIDIA control panel, then go to Display > Adjust desktop size and position, and select Aspect Ratio or No scaling on the Scaling tab.

You may also want to change some of the other options on that page to get a more visually pleasant result; experiment a little.

AMD and Intel have similar options in their driver settings.